1. Field of the Invention
The present invention relates to an image alignment method in checking a pattern of a reticle, and more specifically to an image alignment method for aligning an obtained optical image of the reticle with a reference image to be referred to for checking the pattern of the reticle.
2. Description of the Related Art
A reticle is used to form a predetermined pattern on a semiconductor wafer made of silicone, etc. in an LSI manufacturing process (patterning).
When the pattern of a reticle is defective, the defective pattern of the reticle is transferred on the wafer, and a large volume of defective LSIs are produced. Therefore, it is important to check the pattern of a reticle.
The method for checking the pattern of the reticle for production of LSIs can be either a `die to die check` made by comparing a pattern with the same pattern at a different portion on the same reticle, or a `die to database check` made by comparing the pattern of a reticle with CAD data used when the pattern of the reticle is drawn. The `die to die check` method is disclosed in Japanese Patent Laid-Open No. 4-295748, for example. The `die to database check` method is disclosed in Patent Official Gazette No. 2720935, etc.
A `Die` refers to a pattern area that is a unit for comparison and check of an image pattern, or a detected image. A `database` refers to a reference image synthesized from drawing CAD data in comparison with an optically detected actual pattern image.
In the checking method using a checking device, the check area of a reticle 1 is divided into a plurality of check areas having overlapping portions in the Y direction as shown in FIG. 2. The check areas are sequentially checked, and the defects of all check areas are finally integrated to detect the entire defect of the reticle.
In checking each check area, an XY stage provided with a reticle is first moved to the check starting position.
Then, while moving the XY stage at a constant speed in the X direction, a laser scanning optical device emits a laser beam in the Y direction each time a laser interferometer detects a movement at a predetermined pitch. Then, a transmitted light is detected, and a two-dimensional image is obtained for each frame. The optical image is compared with a reference image synthesized by an image comparison unit to detect a defect. The `frame` refers to a unit of images that can be processed at a time.
When an optical image is obtained using a reticle pattern checking device, the XY stage is moved in the X direction, and the optical image G is sequentially obtained in frame units from the left end to the right end of the reticle 1. However, when the locus of the moving XY stage is curved by an orthogonal error or a mobile line error, and when an optical image obtained in frame units is synthesized, the synthesized image is obtained as curved along the curved locus.
Therefore, the obtained optical image G shows a position shift from the reference image S, and it is necessary to make alignment before comparing images.
However, in the conventional method, a reference alignment is set in a range larger than the obtained optical image G in consideration of the range of the largest possible XY stage error. The obtained optical image G is moved in sub-pixel units within a set range of an alignment area T1. A matching level is obtained between the obtained optical reference image G and the reference image S. According to the common method, the reference image S is compared with the optical reference image G obtained at the highest matching level.
The problem of this method is that the larger the error of the XY stage is, the wider the range in which an optical image is moved becomes. Thereby, the alignment processing time and the checking time on the entire reticle is prolonged.